Oscillating screens are known, used in the separation of solid materials. Known oscillating screens are provided with a screening container of rectangular shape and with a weight of several tonnes, inside which a screening means is disposed, lying on a relative screening plane, inclined by a pre-set angle with respect to a horizontal oscillation plane. The screening container rests on a fixed supporting structure having supporting legs positioned at the corners of the screening container. Each supporting leg has shock-absorber elements made of elastomer material.
The screening container is moved in oscillating manner with respect to the fixed supporting structure by means of a rotary shaft, at an angular velocity comprised between about 100 revs and 300 revs per minute, bearing at a terminal part an eccentric element, with which the lower part of the screening container cooperates. This cooperation between the eccentric element and the screening container occurs by means of suitable mechanical coupling elements.
The rotary shaft is part of a structure having an adjustable counterweight element, acting as a balancing mass, and is moved by means of suitable drive means.
Typically, the eccentric element is disposed in correspondence with the central zone of the lower part of the screening container.
The oscillating movement of the body occurs, in optimum conditions, on a horizontal screening plane, so that the walls of the body remain, during this oscillating motion, in a condition of parallelism with respect to a vertical plane.
One disadvantage of such oscillating screens is that the high loads deriving from stresses of both static and dynamic type due to the oscillation of the container, which has a very great mass, at very high angular velocities, prevent said optimum oscillating conditions from being maintained; that is, they cause unwanted oscillations, that is, outside the horizontal oscillation plane, and also unequal load conditions that vary rapidly over time over the different support elements, reducing the working life thereof.
Said unwanted oscillations may occur on two different and perpendicular vertical planes: one is longitudinal (pitching), and the other is transverse (rolling). These unwanted oscillations cause irreparable damage to the structure of the screening container.
Another disadvantage is that, due to said stresses, a progressive deterioration is determined of the shock absorber elements made of elastomer material disposed on the supporting legs of the screening container. This makes the maintenance of said shock absorber elements particularly expensive, both due to the high cost of the elastomer material used, and also due to the frequent interventions of maintenance and replacement.
A known oscillating screen is described in DE-C-214.999, and is provided with two screening containers lying on four eccentric pins connected to crank shafts and provided with ball supports, disposed in twos along the sides of the container. The eccentric pins both support the screening containers and also determine their oscillating movement.
Another oscillating screen is described in FR-A-323.693, where there is a screening container supported at each of the four corners by supports, in which a rotation shaft and an eccentric are integrated, connected to the relative corner of the container in order to cause it to oscillate. In this case too, the support and oscillation are determined substantially by the elements themselves and in the same position, at the four corners of the container.
The two solutions described above have the disadvantage that they cause excessive mechanical stress to the eccentric mechanisms and consequently they do not ensure a precise rotational and/or oscillating movement.
Another known solution is shown in FR-A-2.685.879, where there is an oscillating screen with a mobile screening container, moved in rotation by a single eccentric mechanism located at one end of the container. The eccentric mechanism is provided with a main central counterweight, mounted on the rotation transmission shaft, and with two counterweights at the sides of the central counterweight, which rotate in the opposite direction, which balance only the centrifugal forces but do not neutralize the tilting torques deriving from the application of said centrifugal forces. Opposite the eccentric mechanism there are suspensions made of stratified elastomers.
Finally, U.S. Pat. No. 3,444,999 shows a free-moving vibrating screen, not controlled by a mechanical eccentric, cushioned by an air chamber, like the tire of an automobile, on which the vibrating body is disposed.
Purpose of the present invention is to achieve an oscillating screen that allows effectively to maintain optimum oscillation conditions and the precise synchronism of the rotational and/or oscillating movement and which, therefore, is not subject to rapid deterioration and/or irreparable damage to its most stressed components, particularly the coupling system between the eccentric element and the screening container and the suspension means associated with the supporting legs and the screening container itself.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.